Ancient stalagmites from a submerged Italian cave have revealed sea level rises caused by global warming more than 200,000 years ago, according to a joint European-Australian study.

The finding, which appears in the journal Nature Geoscience, suggests the current melting of ice sheets may happen faster than expected.

Their publication adds weight to the release of an international report showing up to one-third of all Antarctic sea ice is likely to melt by the end of the century.

Lead author Dr Andrea Dutton, of the Australian National University, says the stalagmites from Argentarola Cave, Italy, provide an ancient archive of sea water levels because they were formed through two different sources.

When the water level in the cave was high, the submerged stalagmites were colonised by aquatic worms that encased it in a tube made from biogenic calcite.

When the level dropped, the stalagmites formed from water drops from the cave ceiling (spelean calcite).

Dutton says the difference between the two types of calcite is stark with the water from the ceiling leaving a darker deposit.

More accurate record

The researchers removed two stalagmites from the cave, one from 18 metres and another from 21 metres below current sea level.

Uranium isotope dating techniques showed the stalagmites recorded water levels in the cave as far back as between 190,000 and 245,000 years ago.

Bracketing of the marine calcite by the spelean calcite also allowed the researchers to accurately gauge the duration of the sea water-level rise.

Dutton says although it was previously known there were three peaks in sea water levels during this time, they "hadn't been well dated".

"Most direct evidence of sea level changes only go back as far as 20,000 years," she says.

Dutton says the strength of the stalagmite archive is that it is not as susceptible to alteration by the environment when compared with coral reef clusters that are also commonly used to date sea water rises.

She says the findings are critical in determining how the natural processes during climate change work.

They provide a benchmark and help determine which reconstruction models used to determine sea water levels are accurate.

"If we are relying on models it is important for us to know which models to rely on," she says. "And this study gives us a benchmark on which to judge them."

Quicker than expected

She says their study shows that "the lag time between temperature change and the sea level rise may not be as long as expected".

By comparing the dates of sea level rises with known increases in temperature, based on studies of ice cores from the Antarctic, they have shown the ice sheets respond rapidly to global warming.

It was previously believed that temperature rise preceded ice sheet melting by more than 3000 years.

"But these two variables have operated in phase for the major termination events in the past 500,000 years," the researchers write in the paper.

Dutton says this suggests ice sheets are capable of responding quickly to increasing temperature and CO2.

That view appears to be supported by the Antarctic Climate Change and the Environment report released this week, which suggests sea temperatures in the Southern Ocean are rising faster than in other oceans.

It predicts ice melts in the Antarctic Peninsula and Western Ice Shelf will be greater and more rapid than expected, highlighting that the Antarctic Peninsula alone has decreased by 27,000 square kilometres in the past 50 years.